You are seeing a natural lighting color distortion/change. It is very common when shooting a black animal to see a blue hue off the fur. Your experiencing the same effect visually off the green of the head.

A couple weeks ago, I took an image of a half dozen Shovelers in flight. 5 drakes show the typical green head. The 6th drake's head looks blue. I mean really blue. Just the way the light was hitting it, I suppose.-Tom

It is most likely light reflection. I was watching a couple just a couple of days ago after seeing this thread and I looked at the shoreline of a lake I was passing and there was a blue headed mallard. I stopped and it suddenly changed direction and his head was green. I am trying to remember but I do believe he was heavily back lit just like yours. When the light hit directly, it was green. Rick

gasrocks wrote in post #7806328Mallards freely mate with many different ducks - especially domestic varieties. I have been told that there are a few Mallards with blue heads out there.

Ditto, I was shooting a few mallards on the lake I live on, and one of them had a blue head. It most definitely had nothing to do with the light. No matter what the angle of light hitting the heads, the green ones were green and the blue one stayed blue. I've also seen some other even more interesting color combinations. Seems those mallards will "get busy" with anybody

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